Injection molding apparatuses, such as hot halves and hot runners, commonly use valve pins to control flow of molding material. Improper valve pin operation can cause damage to the valve pin or to surrounding components, such as a nozzle tip, a nozzle tip liner, a nozzle tip guide sleeve, and/or a mold gate such that it is desirable in certain applications to take a valve pin out of service. In addition, it is sometimes desired to take a valve pin out of service when a mold cavity, a valve pin, a heater, a mold gate, or other related component wears or fails. Replacement of a damaged valve pin or manual disengagement of a valve pin can be time consuming and costly.
Magnetic valve pin couplings have been proposed that permit decoupling of a valve pin from its respective actuator or a common valve pin plate upon experiencing a stopping force. During use of such magnetic valve pin couplings there is contact between a magnet attached to the actuator or valve pin plate and a magnetizable valve pin holder for holding the valve pin. Magnets for use in magnetic valve pin couplings are selected by the pulling force of the magnet, which is the force it takes to pull the magnet free from the valve pin holder or put another way the limit of the holding force of the magnet. The required pulling force of a magnet for a particular injection molding application however is often hard to predict as it depends on the molding application and various factors, for instance one or more of: the properties of the plastic material, such as Durometer hardness, viscosity, and shrink factor; the processing conditions, such as processing temperature, mold temperature, injection speed, and injection pressure; the nozzle design, such as nozzle length, runner diameter, and heat profile of nozzle; the gating style, such as gate seal geometry and material type; the mold design, such as cooling layout and condition; the valve pin, such as its diameter, geometry, and surface roughness; the clearance between the valve pin and its respective valve disk which may vary; and the unknown acceleration of actuation particularly for pneumatic and hydraulic actuations.
Due to space constraints within and temperatures experienced by magnetic valve pin couplings, the selection of affordable magnets of a desirable size and pulling force is limited. If a magnet available for a particular molding application does not have an optimal pulling force, some components of the hot runner system may need to be modified or another type of magnet may need to be purchased and placed into the system to permit the pulling force to be overcome in the particular application so that the magnetic valve pin coupling works as intended. Alternatively, adjustment of a valve pin height for each drop may be performed to have a proper protrusion of the valve pin in the gate area that will result in the pulling force of the magnet being overcome and the magnetic valve pin coupling thereby decoupling as intended. Although these are viable solutions to the limited availability of suitable magnets, there is an ongoing need in the art for a magnetic valve pin coupling that may be used with readily available magnets, and permits simple adjustment of the strength of magnetic attraction between components thereof so that the magnetic valve pin coupling decouples when a stopping force is encountered.